Weathering and Erosion

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Weathering

The process by which rock materials are

broken down by the action of physical or chemical processes.

Mechanical weathering

The breakdown of rock into smaller pieces by physical means.

(Ex. Ice Wedging and Abrasion ).

Agents (Causes) of mechanical weathering include:

ice, wind, water, gravity, plants, and even animals

Frost actionThe alternate freezing and thawing of soil

and rock is a form of mechanical weathering.

One type of frost action, ice wedging. Ice wedging starts when water seeps into cracks, it

freezes during cold weather causing the crack to expand and widen and eventually the rock breaks apart.

Ice wedging

1. Water seeps into cracks during warm weather.

2. Then temperatures drop.

3. The water freezes and expands.

4. Next the ice pushes against the sides of the crack.

5. Then the crack widens.

Abrasion

The grinding and wearing away of rock surfaces

through the mechanical action of other rock or sand

particles.

Agents of mechanical weathering (abrasion)

Wind

Water

Gravity

WaterWhen rocks and pebbles roll along the

bottom of swiftly flowing rivers, they bump into and scrape against each other.

The weathering that occurs eventually causes these rocks to become rounded and smooth.

WindWhen wind blows sand and silt

against exposed rock, the sand eventually wears away the rock’s surface. This is what sandblasting can do to a rock.

Gravity

Abrasion occurs when rocks fall on one another. The force rocks exert on each other as they tumble down a mountainside causes abrasion. (rockslide/rock fall or landslide)

Plants• Plants roots go into existing cracks in

rocks. • As the plant grows, the force of the

expanding root becomes so strong that the crack widens.

• Eventually, the entire rock can split apart, as shown in pictures below

Animals

Animals burrow (dig) through the soil and move soil particles around. This exposes rock to air and water and the rate of weathering increases. (Both Mechanical and Chemical weathering)

Almost any animal that burrows causes mechanical weathering. Ants, earthworms, mice, coyotes, and rabbits are just some of the animals that contribute to weathering.

Moles, insects, earthworms, gophers,

Chemical Weathering

The process that breaks down rocks as a result of chemical reactions is called

chemical weathering.

Common agents of chemical weathering are

water, weak acids, and air.

Acid PrecipitationRain, sleet, or snow, that contains a high

concentration of acids

Acid precipitation can be caused by air pollution from the burning of fossil fuels, such as coal and oil.

Sulfur oxides, nitrogen oxides, and carbon oxides combine with water in the atmosphere and form weak acids, which then fall back to the ground in rain and snow. (Burning of fossil fuels)

Acid in groundwater has weathered limestone to form Rusty’s Cave in Dade County, Georgia.

Acids in Living Things

• Another source of acids that cause weathering might surprise you. Lichens produce acids that can slowly break down rock.

Air

This car is undergoing chemical weathering due to the air. The oxygen in the air is reacting with the iron in the car, causing the car to rust. Water speeds up the process. But the iron would rust even if no water were present. Scientists call this process oxidation.

Oxidation is a chemical reaction in which an element, such as iron, combines with oxygen to form an oxide. This common form of chemical weathering is what causes rust.

Section Summary

• Ice wedging is a form of mechanical weathering in which water seeps into rock cracks and then freezes and expands.

• Wind, water, and gravity cause mechanical weathering by abrasion.

• Animals and plants cause mechanical weathering by turning the soil and breaking apart rocks.

• Water, acids, and air chemically weather rock by weakening the bonds between mineral grains of the rock.

1. In your own words, write a definition for each of the following terms: weathering, mechanical weathering, abrasion, chemical weathering and acid precipitation.

2. Which of the following things cannot cause mechanical weathering?

a. Water

b. Acid

c. Wind

d. animals

3. List three things that cause chemical weathering of rocks.

4. Describe three ways abrasion occurs in nature.

5. Describe the similarity in the ways tree roots and ice mechanically weather rock.

6. Describe five sources of chemical weathering.

7. Making Inferences

Why does acid precipitation weather rocks faster than normal precipitation?

8. Making Comparisons

Compare the weather processes that affect a rock on top of a mountain and a rock buried beneath the ground.

Differential Weathering

Differential weathering is a process by which softer, less weather resistant rocks wear away and leave harder, more weather resistant rocks.

About 50 million years ago, a mass of molten rock cooled and hardened underground to form igneous rock.

The surrounding rock was softer than the igneous rock. As the rocks were weathered for millions of years, the softer rock was completely worn away. The harder, more resistant rock of the tower is all that remains.

The Shape of Rocks

Weathering takes place on the outer surface of rocks. Therefore, the more surface area that is exposed to weathering, the faster the rock will be worn down.

A large rock has a large surface area. But a large rock also has a large volume. Because of the large rock’s volume, the large rock will take a long time to wear down.

If a large rock is broken into smaller fragments, weathering of the rock happens much more quickly. The rate of weathering increases because a smaller rock has more surface area to volume than a larger rock has. So, more of a smaller rock is exposed to the weathering process.

Question 4

How does an increase in surface area affect the rate

of weathering?

Weathering and Climate

Climate is the average weather condition in an area over a long period of time.

Question 5

 Why would a mailbox in a warm, humid climate

experience a higher rate of weathering than a mailbox in a

cold, dry climate?

Temperature and Water

The rate of chemical weathering is faster in warm, humid climates. The rusty mailbox has experienced a type of chemical weathering called oxidation. Oxidation, like other chemical reactions, happens at a faster rate when temperatures are higher and when water is present.

Temperature and Water

Water increases the rate of mechanical weathering.

The freezing of water that seeps into the cracks of rocks is the process of ice wedging. Ice wedging causes rocks to break apart. Over time, this form of weathering can break down even the hardest rocks into soil.

Temperature and Water

Temperature is another major factor in mechanical weathering. The more often temperatures cause freezing and thawing, the more often ice wedging takes place. Therefore, climatic regions that experience frequent freezes and thaws have a greater rate of mechanical weathering.

Question 6

Why do mountaintops weather faster than rocks

at sea level?

Gravity Gravity affects weathering.

The steepness of mountain slopes increases the effects of mechanical and chemical weathering.

Steep slopes cause rainwater to quickly run off the sides of mountains. The rainwater carries the sediment down the mountain’s slope. This continual removal of sediment exposes fresh rock surfaces to the effects of weathering.

Gravity

New rock surfaces are exposed to weathering when gravity causes rocks to fall away from the sides of mountains. The increased surface area means weathering happens at a faster rate.

The effects of weathering can make mountain tops pointed or rounded.

The Appalachian mountains which are older and once taller than the Rockies are now shorter due to mechanical weathering

Section Summary

Hard rocks weather more slowly than softer rocks.

The more surface area of a rock that is exposed to weathering, the faster the rock will be worn down.

Chemical weathering occurs faster in warm, humid climates.

Weathering occurs faster at high elevations because of an increase in ice, rain, and wind.